mtd: onenand: Avoid fall-through warnings

NOTICE THAT:

"...we don't know whether we need fallthroughs or breaks here and this
is just a change to avoid having new warnings when switching to
-Wimplicit-fallthrough but this change might be entirely wrong."[1]

See the original thread of discussion here:

https://lore.kernel.org/patchwork/patch/1036251/

So, in preparation to enabling -Wimplicit-fallthrough, this patch silences
the following warnings:

drivers/mtd/nand/onenand/onenand_base.c: In function ‘onenand_check_features’:
drivers/mtd/nand/onenand/onenand_base.c:3264:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (ONENAND_IS_DDP(this))
      ^
drivers/mtd/nand/onenand/onenand_base.c:3284:2: note: here
  case ONENAND_DEVICE_DENSITY_2Gb:
  ^~~~
drivers/mtd/nand/onenand/onenand_base.c:3288:17: warning: this statement may fall through [-Wimplicit-fallthrough=]
   this->options |= ONENAND_HAS_UNLOCK_ALL;
drivers/mtd/nand/onenand/onenand_base.c:3290:2: note: here
  case ONENAND_DEVICE_DENSITY_1Gb:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Also, notice that this patch doesn't change any functionality. See the
most recent thread of discussion here:

https://lore.kernel.org/patchwork/patch/1077395/

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

[1] https://lore.kernel.org/lkml/20190509085318.34a9d4be@xps13/



Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>